JSON-Parse Failure as Warning should be storred in field "gl2_processing_error"

## Expected Behavior
If the JSON-Parser Functions "parse_json" or "select_jsonpath" in pipeline proceessing is failing due to missing end closures, or missing character quoting or something else. It should be visible in the field "gl2_processing_error".

## Current Behavior
If the JSON-Parser Functions "parse_json" or "select_jsonpath" in pipeline proceessing are failing it shows a a warning which throws the processing error only in the "graylog-server.log". Thoses "warnings/errors" are causing a not processed message which leads in a document missing all the fields.

## Context
If those failures which are despite of causing a lot of failures leadiong to miss processed documents they shpuld be visible in the field "gl2_processing_error" this would be improve the ability to correct the failures and to prevend those "errors"

## Your Environment

* Graylog Version: 3.3.x